Fans are often used to cool electronic equipment. However, failure or reduced speed of the fan can result in overheating and failure or erroneous operation of the fanned electronic equipment. Monitors are often connected to fans in order to detect abnormally slow fan speeds. These monitors provide fan failure signals as an indication that the fanned electronic equipment may not be operating properly.
Bi-metal air flow sensors are typically used as fan speed monitors. These sensors utilize a differential expansion principle in which the air entering an inlet of the sensor, circulates and cools the electrically heated bi-metal material. When the air flow drops below the nominal setting, the termperature of the bi-metal element rises quickly, closing a contact which provides a fan failure signal. Typical examples of such sensors are the series LS air flow sensors from the Warren Communications division of General Signal Corporation. However, such air flow sensors also generate heat and their response time varies, depending on the monitored air velocity. In addition the preset or nominal air flow thresholds of such sensors are limited and inflexible.